Fix return type on the cart price rule creation BO page:

Type Error occurred when creating object: Magento\SalesRule\Model\Rule\Condition\Product\Interceptor, Smile\CustomEntityProductLink\Plugin\Model\ResourceModel\Eav\Attribute::afterIsAllowedForRuleCondition(): Return value must be of type bool, string returned

Plugin/Model/ResourceModel/Eav/Attribute.php

@@ -13,8 +13,8 @@ class Attribute
      */
     public function afterIsAllowedForRuleCondition(Subject $subject, bool $result): bool
     {
-        if ($subject->getFrontendInput() == 'smile_custom_entity') {
-            $result = $subject->getIsVisible();
+        if ($subject->getFrontendInput() === 'smile_custom_entity') {
+            $result = (bool) $subject->getIsVisible();
         }
 
         return $result;